Apparatus and method for synthesizing films and coatings by focused particle beam deposition

ABSTRACT

A particle beam deposition apparatus includes a particle source for generating a plurality of particles in suspended form, an expansion chamber, and a deposition chamber connected to the expansion chamber by an aerodynamic focusing stage, and containing a substrate. The aerodynamic focusing stage may be comprised of a plurality of aerodynamic focusing elements, or lenses. Particles, including nanoparticles, may be deposited on the substrate by generating an aerosol cloud of particles, accelerating the particles into the expansion chamber, creating a collimated beam out of the particles by passing them through the aerodynamic focusing lenses and into a deposition chamber, and impacting the particles into the substrate.

STATEMENT AS TO FEDERALLY-SPONSORED RESEARCH

[0001] Funding for work described herein was provided in part by thefederal government, which may have certain rights in the invention.

TECHNICAL FIELD

[0002] This invention relates to deposition of particles on a substrate,and more particularly to deposition of nanoparticles by focused particlebeam deposition to form films or coatings.

BACKGROUND

[0003] The synthesis and processing of nanostructured materials, i.e.,materials with grain sizes less than about 100 nm, is of great interestbecause such materials are known to have properties different from, andoften superior to, those of conventional bulk materials. Theseadvantages include greater strength, hardness, ductility, andsinterability, size-dependent light absorption, and greater reactivity.Applications for these advanced materials include ductile ceramics,wear-resistant coatings, thermal barrier coatings, new electronic andoptical devices, and catalysts. There has been considerable progress indetermining the properties of nanostructured materials, small amounts ofwhich have been synthesized (mainly as nanosize powders), by processessuch as colloidal precipitation, mechanical grinding, and gas-phasenucleation and growth. The focus of more recent research has been toproduce nanostructured materials directly in a form suitable for use ina practical application, such as wear-resistant coatings. In suchmaterials, the nanostructure is deliberately introduced to takeadvantage of superior properties, for example, enhanced hardness in thecase of nanostructured wear-resistant coatings.

[0004] The current interest in nanostructured materials has led to thesearch for methods to synthesize such materials in the form of bulksolids or films. The production of nanostructured materials hasgenerally involved two or more steps, including the controlled synthesisof nanosize powders, and the assembly of these powders intonanostructured materials by sintering or other means.

[0005] Gas-phase nucleation and growth of particles is an establishedroute for the synthesis of nanosized powders and includes suchtechniques as evaporation-condensation, laser pyrolysis, and thermalplasma expansion processing. Gleiter, H., “Nanocrystalline Materials,”Prog. Mater. Sci. 33:223-315 (1989); Recknagle, K. et al., “Design andOperation of a Nanocluster Generation and Collection System,” AerosolSci. Technol. 22:3-10 (1995); Oda, M. et al., “Ultrafine particle filmsby gas deposition method,” Mat. Res. Soc. Symp. Proc. 286:121-130(1993); Flint, J. H. et al., “Powder temperature, size and numberdensity in laser-driven reactions,” Aerosol Sci. Technol. 5:249-260(1986); Rao, N. et al., “Nanoparticle formation using a plasma expansionprocess,” Plasma Chem. Plasma Proc., 15:581-606 (1995). In many of thesegas-phase processes, the nanosize powders are collectedthermophoretically and consolidated in-situ using high pressurecompaction to produce pellets of nanostructured materials. Gleiter, H.,“Nanocrystalline Materials,” Prog. Mater. Sci. 33:223-315 (1989);Recknagle, K. et al., “Design and Operation of a Nanocluster Generationand Collection System,” Aerosol Sci. Technol. 22:3-10 (1995).

[0006] The use of inertial impaction provides a convenient route forassembling particles into consolidated materials, includingnanoparticles into nanostructured materials. That is because heavyparticles seeded in a light carrier gas can be deposited efficiently byaccelerating the gas through a nozzle, preferably into a low pressureregion, and directing the resulting high speed aerosol jet against adeposition substrate. Fernandez de la Mora, J., “Surface impact ofseeded jets at relatively large background densities,” J. Chem. Phys.82:3453-3464 (1985); Fernandez de la Mora et al., “Hypersonic impactionof ultrafine particles,” J. Aerosol Sci. 21:169-187 (1990). Atsufficiently low pressures, even though the host gas may deceleratebefore impacting the substrate, e.g., by formation of a shock, the heavyparticles continue their forward motion and impact by virtue of theirgreater inertia.

[0007] Until recently, high-speed impaction had been used mainly forparticle measurement. More recently, however, a number of materialsdeposition processes based on this principle have been developed,including those that deposit heavy molecules, ultra fine particles, andlarge micron-sized particles. Schmitt, J. J., “Method and apparatus forthe deposition of solid films of a material from a jet stream entrainingthe gaseous phase of said material,” U.S. Pat. No. 4,788,082 (1988);Halpern, B. L. et al., “Gas jet deposition of thin films,” Appl. Surf.Sci. 48/49:19-26 (1991); Calcote, H. F. et al., “A new gas-phasecombustion synthesis process for pure metals, alloys, and ceramics,”24th Symp. (Intl.) on Combustion, Combustion Inst. Pittsburgh 1869-76(1992); Kashu, S. et al., “Deposition of ultrafine particles using a gasjet,” Jap. J. Appl. Phys. 23:L910-912 (1984); Oda, M. et al., “Ultrafineparticle films by gas deposition method,” Mat. Res. Soc. Symp. Proc.286:121-130 (1993); Gould, R. K. et al., “Apparatus for producing highpurity silicon from flames of sodium and silicon tetrachloride,” U.S.Pat. No. 5,021,221 (1991).

[0008] Some of the inventors of this disclosure have themselvesparticipated in developing a process, hypersonic plasma particledeposition (HPPD), for the inertial deposition of nanoparticles to formnanostructured films. In HPPD, vapor-phase reactants are injected into athermal plasma, which is then expanded to low pressure through a nozzle.Rapid cooling in the nozzle expansion drives the nucleation ofnanoparticles, which are then accelerated in the hypersonic free jetissuing from the nozzle. A substrate may be positioned normal to theflow, and particles as small as a few nanometers in diameter deposit byinertial impaction. Ballistic compaction forms a dense, nanostructuredcoating. In experiments involving silicon carbide deposition, the grainsize observed by scanning electron microscopy (SEM), typically around 20nm, corresponded closely to measurements by scanning electrical mobilityspectrometry of the aerosol sampled in-flight downstream of the nozzle,indicating that the film retained the grain size of the impactingparticles. Rao, N. P. et al., “Nanostructured materials production byhypersonic plasma particle deposition,” Nanostructured Materials,9:129-132 (1997); Rao, N. P. et al., “Hypersonic Plasma ParticleDeposition of Nanostructured Silicon and Silicon Carbide,” J. AerosolSci., 29:707-720 (1998); Rao, N. et al., “Plasma chem. Plasma Process15, 581 (1995); Neumann, A. et al., “J. Nanoparticle Res., accepted forpublication January 1999; Blum, J. et al., “Nanoparticle Research,” 1,31 (1999). A patent on the HPPD process, U.S. Pat. No. 5,874,134,assigned to the Regents of the University of Minnesota, is herebyincorporated by reference.

[0009] Nanoparticle deposition processes have recently been used tocreate patterned films without the use of masking through the use ofcollimated beams of nanoparticles. Most of this work has been directedat depositing metal patterns for printed circuit board and electronicsapplications. Schroth, A. et al., Jpn. J. Appl. Phys., 37, 5342 (1998);Akedo, J. et al., Jpn. J. Appl. Phys., 38:5397 (1999); Akedo, M. et al.,“Sensors Actuators,” A 69:106 (1998). For example, the gas jetdeposition (GJD) process has been used to “write” metal patterns fordepositing gold or silver particles generated by inert gas-condensation.Hayashi, C. et al., “The use of nanoparticles as coatings,” MaterialsSci. Eng. A163:157-161 (1993). In this method, condensable vapor isgenerated above a heated crucible and particles nucleate in an inertcarrier gas. The particle-laden flow then expands supersonically througha micronozzle, producing a particle beam whose dimensions areapproximately the same as, or perhaps somewhat smaller than, those ofthe nozzle. Nozzles with inside diameters of 100 μm were used to depositgold particles, producing tapered needle-shaped structures. Severaldeposition nozzle designs were tested, including rectangular slit-typegeometric and multinozzle assemblies.

[0010] Similar techniques have been used to create patterned films bydepositing iron nanoparticles, and more recently to fabricate highaspect ratio structures for micro-electro-mechanical systems (MEMS)applications. A variety of techniques were used, including free forming,insert-molding, and substrate masking. To suppress clogging, the nozzleswere heated, so as to drive particles away from the nozzle walls bythermophoretic forces. Representative techniques are described inKizaki, Y. et al., “Ultrafine Particle Beam Deposition I. Sampling andTransportation Methods for Ultrafine Particles,” Jpn. J. Appl. Phys.Vol. 32:5163-5169 (1993); Akedo, J. et al., “Fabrication of ThreeDimensional Micro Structure Composed of Different Materials UsingExcimer Laser Ablation and Jet Molding,” Proceedings of the 10th AnnualInternational Workshop on Micro Electro Mechanical systems, Nagoya,Japan, 135-140, Jan. 26-30, 1997.

[0011] Although the GJD process has aspects similar to HPPD, the processconditions used in GJD are significantly different, with a relativelyhigh particle source chamber pressures (−0 Torr −5 atm.), necessitatingthe use of light carrier gases (e.g., helium) and small nozzledimensions (−100 μm) to achieve inertial deposition of nanoparticles.Deposition rates for GJD are on the order of 10 μm/min, with depositdimensions close to that of the nozzle, though the sharpness of thepattern is somewhat limited by the presence of a broad “tail”surrounding the core deposit.

[0012] The GJD method is limited in its ability to produce very smallfeatures. Pattern feature dimensions have been decreased down to 40 μmby suitably reducing nozzle dimensions. Reducing the nozzle size,however, greatly diminishes the deposition rate, since the gas flow ratevaries as the square of the nozzle diameter. In addition, the use ofsmall nozzles makes it more difficult to control the nozzle-to-substratedistance precisely, which in the GJD process scales with the nozzlediameter and determines the cut-size of impacting particles. Smallernozzles are also difficult to manufacture with precision and are moresusceptible to clogging at high particle loading.

[0013] Particle beams in areas other than nanoparticles have beencontrolled and focused using aerodynamic lenses. Aerodynamic focusing isdiscussed by Dahneke, B. et al., “Similarity theory for aerosol beams,”J. Colloid Interface Sci., 87, 167-179 (1982); Fernandez de la Mora etal., “Aerodynamic focusing of particles and molecules in seededsupersonic jets,” in Rarefied Gas dynamics: Physical Phenomena, [editedby Muntz, E. P., weaver, D. P. and Campbell, D. H.], Vol. 117 ofProgress in Astronautics and Aeronautics, AIAA, Washington D.C., 247-277(1989), and aerodynamic lenses are described by Liu, P. et al.,“Generating particle beams of controlled dimensions and divergence: I.Theory of particle motion in aerodynamic lenses and nozzle expansions,”Aerosol Sci. Technol. 22:293-313 (1995); Liu, P. et al., “Generatingparticle beams of controlled dimensions and divergence: II. Experimentalevaluation of particle motion in aerodynamic lenses and nozzleexpansions,” Aerosol Sci. Technol. 22:314-324 (1995). In aerodynamicfocusing, particles may be shaped into a narrow beam by passing theaerosol through a series of constrictions (aerodynamic lenses). The gasundergoes a converging and diverging motion as it flows through thelenses. Due to inertia, particles seeded in the flow may eitherconcentrate along the flow axis or be deposited on the walls of thefocusing system, depending on their size. For a given lens geometry, gascomposition, particle composition, and gas flow rate, this tendency tofocus depends strongly on the particle size. Particles of a certaincritical size are pushed to the axis, while subcritical particles show aconcentrating tendency that decreases with particle size. Particleslarger than critical overshoot the focal point and are removed from theflow by collision with walls. With a single lens, only particles withina very narrow range of particle sizes are focused. Particles within abroader size range can to be focused by using a set of focusing lenseswith gradually decreasing diameters. Typically, five lenses may beneeded to cover one order of magnitude in particle size. The lenses maybe used in combination with a downstream supersonic nozzle to accelerateand deposit particles in a specified range of particle sizes.Aerodynamic focusing is discussed in U.S. Pat. No. 5,270,542,incorporated herein by reference.

SUMMARY

[0014] A new technology is described herein for depositing patternedfilms and coatings in general, and functional nanostructures such asnanostructured patterned films and coatings in particular. Thetechnology is based on the generation of gas-borne particles, such asnanoparticles from a thermal plasma expansion reactor, confining theparticles in a narrow, high-speed particle beam by passing the aerosolflow through an aerodynamic focusing stage, followed by high-speedimpaction of the tightly focused particles onto a substrate in a vacuumdeposition chamber. The focusing stage may consist of one or morefocusing. elements (aerodynamic lenses) and a nozzle that may beintegrated into the inlet of the deposition chamber. The aerodynamiclenses may focus the particles emerging from the aerosol source into anarrow beam, while the nozzle accelerates the focused particles toterminal velocities on the order of the sonic speed of the carrier gas.The particles in the beam may be deposited by inertial impaction ontothe substrate in the evacuated deposition chamber, resulting in theformation of a consolidated deposit. Patterned films may be deposited bytranslating the substrate with respect to the nozzle, or vice versa.

[0015] Nanostructured patterned films of various materials may bedeposited in this manner using an aerosol source wherein the generatedparticles are nanosized (i.e., with diameters<100 nm). Nanostructuredpatterns of materials such as silicon carbide may be used inapplications such as wear-resistant coatings for MEMS or micro-fluidicssystems and devices. Line widths of less than 50 microns can beproduced. This approach can permit the use of much larger nozzles thanin previously developed micronozzle methods, and also can permitsize-selection of the particles that are deposited.

[0016] The details of one or more embodiments of the invention are setforth in the accompanying drawings and the description below. Otherfeatures, objects, and advantages of the invention will be apparent fromthe description and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

[0017]FIG. 1 is a schematic diagram of a system for depositingnanoparticles on a substrate via a collimated beam.

[0018]FIG. 2 is a schematic diagram of another system for depositingnanoparticles on a substrate via a collimated beam.

[0019]FIG. 3a is a diagram of predicted gas streamlines in ananoparticle focused beam deposition system.

[0020]FIG. 3b is a diagram of predicted particle trajectories in ananoparticle focused beam deposition system.

[0021]FIG. 4 is a scanning electron microscope image of a needle-shapedsilicon carbide tower produced by nanoparticle focused beam deposition.

[0022]FIG. 5 is a scanning electron microscope image of a pattern ofsilicon carbide produced by nanoparticle focused beam deposition.

[0023]FIG. 6 is a schematic diagram of a system for depositingnanoparticles on a substrate via a collimated beam.

[0024]FIG. 7a is a diagram of predicted velocity vectors and Machnumbers in a hypersonic particle beam deposition system.

[0025]FIG. 7b is a diagram of predicted temperature contours in ahypersonic particle beam deposition system.

[0026]FIG. 8 is a graph comparing the RBS spectrum for a deposited filmof SiC to a spectrum for standard SiC.

[0027]FIG. 9 is a graph that compares hardness for a deposited film ofSiC to that of standard SiC.

DETAILED DESCRIPTION

[0028] A novel technology, termed focused particle beam deposition(FPBD), is capable of depositing patterned films, includingnanostructured patterned films, with higher deposition rates and sharperdefinition than that possible with the prior art processes. One type ofapparatus for practicing FPBD is schematically depicted in FIG. 1. Inthe apparatus 10, gas-borne particles are generated in a source 12 suchas a plasma expansion reactor. The particles are transported by the flowinto an intermediate expansion chamber 16, which is maintained at apressure, typically below 10 torr, by a mechanical vacuum pump 18. Theexpansion chamber 16 may communicate with a downstream depositionchamber 20 through a focusing inlet 24. A portion of the expanded flowfrom the aerosol source may enter the deposition chamber 20 through aninlet 24 provided with aerodynamic focusing lenses 26 and anaccelerating nozzle 28. The deposition chamber 20 may be evacuated by avacuum pump 30, such as a turbo pump, and maintained at a low pressure,e.g., on the order of 0.01 torr or lower in the case of a turbo pump.

[0029] The particles in the flow entering this final chamber 20 may befocused into a narrow beam and then impacted at high speeds against asubstrate 32 placed within the chamber 20, at a distance within 1-100nozzle diameters downstream of the inlet 24. Patterned films may bedeposited by translating the substrate 32 in relation to the nozzle, orvice-versa. The differential pumping arrangement of the type shown inFIG. 1 is flexible and may permit independent control of aerosol sourceflow and deposition flow rates. Flexibility in setting the pressure ofthe intermediate chamber may provide a means by which to apply the FPBDprocess over a broad range of particle sizes. In addition, the aerosolexpansion chamber could be omitted under certain conditions.

[0030] The focusing assembly 22 may consist of a series of thin plates34 mounted in a cylindrical barrel 36. Each plate 34 has an orifice, or“aerodynamic lens,” located at its center, and the assembly 22 mayterminate in an exit nozzle 24, which is a sonic orifice. In passingthrough each lens 26, the streamlines of the gas flow contract andre-expand. Very small particles follow the gas streamlines. Very largeparticles accelerate radially toward the axis as the flow approaches theorifice. Due to their high inertia, these particles are projected acrossthe centerline and impact on the opposite wall of the focusing assembly.Particles of an intermediate size are also accelerated toward the axis,but due to their shorter aerodynamic stopping distance, they terminatetheir radial motion on a flow streamline that is closer to the axis thanthe one on which they originated. This may lead to a concentration ofsuch particles along the axis. By careful design of lens assembly 22, itis possible to collimate particles having a specified range of sizes.

[0031] The use of aerodynamic lenses upstream of the nozzle may enablethe deposition of entrained particles within a tightly confined areawhose diameter is substantially narrower than that of the nozzle, e.g.,by an order of magnitude or more. As a result, for equivalent nozzlesizes, the use of aerodynamic focusing may enable the deposition ofpattern features with dimensions up to 10 sizes smaller than thatpossible with conventional gas jet deposition without the use of lenses.Alternatively, for equivalent feature dimensions and source particleconcentrations, the use of aerodynamic focusing can permit depositionrates (i.e., “pattern writing speeds”) 100 times greater than thatpossible in conventional gas jet deposition without focusing. Theminimum attainable beam diameter depends on practical design issues suchas tolerances in lens alignment and the residual thermal motion ofparticles exiting the nozzle. Small particles still retain a componentof thermal velocity in the radial direction which causes the focusedbeam to broaden downstream of the nozzle. The beam diameter increaseswith decreasing particle size and increasing beam path length. For abeam of 20 nm particles, the beam divergence over a nozzle-to-substrateseparation distance of 5 mm may be on the order of 10 μm. This isrelatively low, and it is likely that factors such as tolerances in lensalignment will have a greater effect on the minimum attainable featuresize.

[0032] In one embodiment of the system, shown in FIG. 2, particles inthe sub-100 nm range may be generated in a nanoparticle source 40comprising a plasma expansion reactor. The in-flight agglomeration ofthe particles may be minimized by hypersonic transport into anintermediate expansion chamber 42. The particles may then be confinedinto a narrow beam by passage through a focusing inlet 44 provided withaerodynamic lenses 46, communicated with a downstream vacuum depositionchamber 48, and deposited onto a translating substrate 50. Thenanoparticle source 40 preferably comprises a dc torch 52 with a ceramicflow passage 54 mounted downstream, into which reactant feedstock,preferably in gas- or vapor-phase, is injected. The pressure in theinjection region is preferably in the range 300-750 torr, but may be anyappropriate value. Nanoparticles are nucleated by expanding the hotreacting gases through a ceramic-lined nozzle 56, preferably having exitdiameters in the range 1-10 mm. The gas temperature at the nozzleentrance is preferably between 2000K and 5000K, and that at the nozzleexit between 1000K and 2500K. The expansion chamber is preferably at apressure below 10 torr. and is pumped by a large Roots Blower (orequivalent) mechanical vacuum pump 58. Flow rates through the reactorare preferably in the range 1-100 slpm.

[0033] The focusing inlet 44 communicating with the final depositionchamber 48 may have multiple lens elements 46 to focus a broad range ofparticle sizes. In certain embodiments, it may be preferable to havethree to five lens elements. The inlet 44 may be provided with anaccelerating nozzle which may be operated under choked flow toaccelerate the particles previously focused. The diameters of theaerodynamic lens elements 46 are selected preferably to focus particleswithin the sub-100 nm diameter range. The diameters of the lens elements46 and the nozzle preferably range from 0.1 to 5 mm. The actual valuesdepend on process parameters such as carrier gas composition, expansionchamber temperature and pressure, and particle aerodynamic diameter. Thenozzle-to-deposition substrate distance is preferably in the range of1-100 nozzle diameters.

[0034] The system and process are not limited to sources comprising athermal plasma reactor alone. The FPBD process is also not limited toparticles generated by chemical reaction alone, but is also applicableto particles generated by physical evaporation and condensation, e.g.,starting from feedstock materials including solids, liquids, and gases;and by electrospray processing. The FPBD concept can be applied ingeneral to any source of gas-borne nanoparticles, including laserpyrolysis reactors, evaporation-condensation reactors, or electrosprayatomizers, in processes in which the object of processing is thedeposition of gas-borne nanoparticles onto substrates to form patternedfilms of various materials, such as metals, inorganic materials (e.g.,ceramic oxides, carbides, nitrides, and mixtures of the same), andorganic materials (e.g., polymers and plastics). The FPBD systemincorporating a nanoparticle source can be used to depositnanostructured patterned films, which may possess unique and enhancedproperties when compared to films of more conventional materials.

[0035] Experiments were conducted in which either SiC or titaniumnanoparticles were generated. An argon-hydrogen plasma was generated bya direct-current arc. Reactants were injected into the plasma at theupstream end of the expansion nozzle. The reactants consisted either ofsilicon tetrachloride and methane, for silicon carbide synthesis, ortitanium tetrachloride, for titanium synthesis. The pressure wasapproximately 50 kPa at the nozzle inlet and 345 Pa in the expansionchamber. The plasma is hot (approximately 2000 K) at the nozzle exit andexpands supersonically into the large expansion chamber. The inlet tubeto the aerodynamic lens assembly was coaxially located 75 cm downstreamof the plasma expansion nozzle. The flow in the expansion chamberexperienced a series of expansion and compression waves, and is expectedto have relaxed to close to room temperature at the inlet of the lensassembly.

[0036] The lens assembly consisted of a series of five lenses, each withan orifice diameter of 2.26 mm. The inner diameter of the exit nozzlewas 1.85 mm. Each lens was 0.3 mm thick, and the lenses were spaced 47mm apart. The entire unit was constructed of stainless steel.

[0037] The particle beam exiting the lens assembly issued into a chamberthat was maintained at a pressure of 1.0 Pa. Substrates were mounted inthis chamber, typically 3 mm downstream of the exit nozzle. The particleimpact velocity was estimated to be in the range 200-300 m/s. Thesubstrates were at room temperature. Various substrate materials wereused, including stainless steel, aluminum, brass and glass. Adherentdeposits formed on all of the metal substrates, but not on glass.

[0038] Numerical simulations were conducted to predict the flow ofcarrier gas and the trajectories of (assumed spherical) silicon carbideparticles of various sizes through a lens system with the same geometryand conditions as in the experiments. These simulations solved theconservation equations for steady, laminar, compressible flow, andcalculated particle trajectories accounting for viscous drag but notBrownian diffusion. Brownian diffusion would be expected to broaden thewidth of the focused beam, especially for particles smaller than about10 nm in diameter. The predicted gas streamlines and trajectories of20-nm-diameter particles that enter the lens assembly along variousstreamlines are shown in FIGS. 3a and 3 b, respectively. The particlesare predicted to be well collimated along the flow axis by the exit ofthe final lens.

[0039] An SEM image of a needle-shaped silicon carbide tower depositedon a stationary substrate produced by FPBD is shown in FIG. 4. Theheight of this structure is 1.3 mm. The compact, tapered appearance istypical for both SiC and titanium. High-resolution SEM images obtainedfrom cross-sections of the titanium deposits show grain sizes of about20 nm, similar to those previously reported for HPPD of silicon carbide.In general, the height, half-width, and base diameters of the towersincrease linearly with deposition time, and the dimensions are similarto those using micronozzles. Kashu, S. et al., Jpn. J. Appl. Phys.23:L910 (1984); Oda, M. et al., MRS Symp. Proc. 286:121 (1993); Schroth,A. et al., Jpn. J. Appl. Phys. 37:5342 (1998); Akedo, J. et al., Jpn. J.Appl. Phys. 38:5397 (1999); Akedo, J. et al., Sensors Actuators A 69:106(1998). However, aerodynamic focusing may allow one to use much largernozzles to achieve the equivalent results. Because a ten-fold increasein nozzle diameter corresponds to a hundred-fold increase in flowrate,aerodynamic focusing may allow much higher throughputs. In addition,although nozzle clogging may still be an issue with the present system,the use of larger nozzles should help alleviate the problem.

[0040] Experiments have demonstrated the feasibility of depositing linesand two-dimensional patterns by translating the substrate. A patternformed by SiC particles is shown in FIG. 5. The substrate was translatedmanually in a rapid zig-zag motion. The minimum line width is about 50μm. As can be seen in the figure, towers began to grow at several pointswhen the translation momentarily paused. An automated x-y translationsystem has also been implemented and has been used to deposit titaniumlines, with a width of about 30 to 50 μm. Lower Si precursor feeds inthe plasma reactor improve operation characteristics and produce cleanernanoparticle streams. FIGS. 4 and 5 are not very clear—image quality isvery poor.

[0041] Downstream of the lenses, a critical nozzle forces hypersonicdeposition of the particles. The dimensions of that nozzle define thefinal feature size of the patterns. Beams with diameters in the order oftens of microns may be produced.

[0042] Another embodiment, which can be used at relatively low flowrates, is shown schematically in FIG. 6. Its principle component is ahigh-temperature plasma reactor 60 with an injection ring 62 and anozzle 64. The injection ring 62 and nozzle 64 may be constructed from asingle piece of boron nitride. Reactants (e.g., SiCl₄ and CH₄) may beinjected into the plasma, at a location where the temperature is highenough (e.g., >4000K) to promote virtually complete disassociation.Inside the nozzle 64, rapid quenching causes particles to nucleate.These particles may then be accelerated with the flow to supersonicvelocities. The reactant feed system 66 may include a bubbler system 68to gasify the SiCl₄ liquid precursor.

[0043] Substrate temperature control may be achieved by a cooling system(not shown) which may combine water and Ar/He feed, according to thedesign of Bieberich and Girshick. Bierberich, M. T., and Girshick, S.L., Plasma Chem. Plasma Process. 16:157S (1996). A second chamber 70containing the substrate 72 may be pumped down to −10³ Torr by aturbomolecular pump 74, to assist in hypersonic expansion through thecritical orifice 76 located after the aerodynamic lenses 78.

[0044] Computations for a two-dimensional asymmetric flow of pure argonwere conducted. The computations simulate flow through the nozzle,including detailed heat transfer through the boron nitride nozzle walls,and the flow between the nozzle exit and the substrate. A uniformvelocity inlet condition was assumed, and the gas temperature at thenozzle inlet was set to 4000K. Other boundary conditions included apressure of 2.5 Torr at the exit of the flow field, and convective andradiative heat losses from the nozzle's outer boundary. The simulationswere performed using the commercially available software CFD-ACE. Aconjugate gradient solver was used to make calculations across flowboundaries. A two-dimensional plot of calculated velocity vectors andMach numbers is shown in FIG. 7a, and predicted temperature contours areshown in FIG. 7b. Mach numbers up to 6.5 are predicted. A detachedrecompression shock is predicted to be located about 2.5 mm above thesubstrate.

[0045] While little materials characterization has been performed on thefilms deposited by focused deposition, the properties are expected to bevery similar to those obtained in the hypersonic plasma particledeposition process. These properties are described in the following.

[0046] In an SEM image, the deposited films appear to have the samemorphology as those deposited earlier using higher Si precursor flowrates, with the exception of the absence of macroparticles in the newresults. Rutherford back scattering (RBS) analysis showed that highsubstrate temperature (740° C.) reduced the chlorine content of the filmto below 1.5%. Film density was determined by RBS to be 80% of thetheoretical SiC density for the 740° C. substrate temperatureexperiments. Density measurements indicated porosity increasing withdecreasing substrate temperature. FIG. 8 shows a comparison of the RBSspectrum for a deposited film to a spectrum for standard SiC.

[0047] Mechanical properties of films deposited at substratetemperatures of 450° C. and 700° C. were evaluated with thenanoindentation method. Film thicknesses were approximately 5-6 μm.Indentation tests were carried out with two different nanoindenters, aHysitron Triboscope and a micromechanical tester (MMT). The HysitronTriboscope operating in conjunction with an atomic force microscopecombines nanoindentation with the ability to image the indented area.With the MMT, higher loads (up to 900 mN) can be achieved compared tothose available with the Hysitron (a few mN). The nanoindentation testswere performed with conical 90-degree indenters having radii of 400 nmand 1 μm for the Hysitron and MMT, respectively. Hardness evaluationrelied on load-displacement curve analysis using the Oliver and Pharrmethod. Oliver, W. C., and Pharr, G. M., J. Mater. Res. 7:1564 (1992).

[0048]FIG. 9 summarizes the hardness measurements. From this figure, itis evident that hardness values are higher for the high-temperaturedeposit. Even at the relatively high level of 20% porosity, these valuescorrespond to the top of the hardness range reported for fully densestandard SiC. For both films, measurements at shallow penetration depths(Hysitron) indicate that hardness decreases with decreasing depth ofpenetration. This result is consistent with the high surface roughnessand elevated porosity of near-surface layers (observed by SEM).Indentation with the MMT at greater penetration depths yieldsdepth-independent values of hardness. These values are lower than thoseexpected from the low-depth measurement trends.

[0049] Qualitative adhesion strength assessment was performed with a 1kg applied load using a Vickers Indenter. Higher temperature depositsexhibited better adhesion, and only film cracking was apparent. Incontrast, extensive spalling was observed for a lower temperaturedeposit. Quantitative measurements were not possible in either case, dueto irregular spalling or film cracking. To obtain quantitative adhesionmeasurements, 1 μm tungsten overlayer was sputtered over the SiC films.Tungsten was expected to reduce the effects of indentation-inducedcracking and to increase the force for delamination. Indentation teststo evaluate adhesion were carried out with the MMT. For a lowtemperature film, measurements obtained from delamination induced by a900 mN load indicate an adhesion strength of 2.7-3.9 J/m². Nodelamination was observed for the high temperature film.

[0050] A number of embodiments of the invention have been described.Nevertheless, it will be understood that various modifications may bemade without departing from the spirit and scope of the invention.Accordingly, other embodiments are within the scope of the followingclaims.

What is claimed is:
 1. A particle deposition apparatus, comprising: a) aparticle source that generates a plurality of particles in a suspendedform; b) an expansion chamber in communication with the particle sourceand having an interior pressure that is substantially lower than theinterior pressure of the particle source; c) a deposition chamber; d) asubstrate support located inside the deposition chamber; and e) anaerodynamic focusing stage connecting the expansion chamber to thedeposition chamber, and comprising a plurality of aerodynamic focusingelements.
 2. The particle deposition apparatus of claim 1, wherein theparticle source generates a plurality of nanoparticles.
 3. The particledeposition apparatus of claim1, wherein the particle source includes adc torch.
 4. The particle deposition apparatus of claim1, wherein theparticle source is a thermal plasma expansion reactor.
 5. The particledeposition apparatus of claim1, wherein the particle source is a laserpyrolisis reactor.
 6. The particle deposition apparatus of claim1,wherein the particle source is an evaporation-condensation reactor 7.The particle deposition apparatus of claim1, wherein the particle sourceis an electrospray particle source.
 8. The particle deposition apparatusof claim1, wherein the aerodynamic focusing stage comprises between twoand five aerodynamic focusing elements.
 9. The particle depositionapparatus of claim1, further comprising a hollow nozzle between theparticle source and the expansion chamber.
 10. The particle depositionapparatus of claim 9, wherein the hollow nozzle generates a hypersonicflow of particles, and the aerodynamic focusing stage focuses the flowof particles into a collimated beam.
 11. The particle depositionapparatus of claim 10, wherein the substrate support is positionednormal to the collimated beam.
 12. he particle deposition apparatus ofclaim 1, wherein the substrate support is translatable.
 13. A method fordepositing particles on a substrate, comprising the steps of: a)generating an aerosol cloud of particles; b) accelerating the particlesthrough a nozzle; c) creating a collimated beam of particles by passingthe particle through a plurality of aerodynamic focusing lenses; d)impacting the collimated beam of particles against the substrate. 14.The method of claim 13, wherein the particles are comprised ofnanoparticles.
 15. The method of claim 13, wherein the aerosol cloud isgenerated in a plasma expansion reactor.
 16. The method of claim 13,wherein the nanoparticles are accelerated through the nozzle tohypersonic speeds.
 17. The method of claim 13, further including thestep of translating the substrate in a predetermined manner to create apattern of nanoparticles on the substrate.
 18. The method of claim 17,wherein the substrate is translated in more than one direction.
 19. Aparticle structure comprised of particles, formed by the steps of: a)generating an aerosol cloud of particles; b) accelerating the particlesthrough a nozzle; c) creating a collimated beam of particles by passingthe particles through a plurality of aerodynamic focusing lenses; d)impacting the collimated beam of particles against a substrate.
 20. Theparticle structure of claim 19, wherein the particles are comprised ofsilicon carbide.
 21. The particle structure of claim 19, wherein theparticles form a functional structure.
 22. The particle structure ofclaim 19, wherein the particles form a protective coating
 23. Theparticles structure of claim 19, wherein the particles are comprised ofnanoparticles.
 24. The particles structure of claim 19, wherein theparticles consist essentially of nanoparticles.